1. Field of the Invention:
The present invention relates to a motor driving circuit preferable for motor control.
2. Description of the Related Art:
FIG. 2 is a circuitry diagram showing a driving section of a three phase motor.
A brushless motor comprises a rotor fixed a rotation axis (shaft) so as to oppose to a stator. U-phase, V-phase, W-phase driving coils 1, 2, 3, which are Y-connected to one another, are fixedly connected to the stator. Magnets are fixed to the rotor to form a plurality of poles. Hall elements are fixed to the stator. When the magnets pass above the driving coils 1, 2, 3 as the motor rotates, three sinusoidal wave signals, indicative of variation of a magnetic field, have a phase difference among one another of a 120xc2x0 electrical degree are obtained by the Hall elements. NPN-type source-side transistor 4U and sink-side transistor 5U are connected in series between the power source Vcc and the ground Vss, and an emitter-collector connection therebetween is connected to the released-side end of the U-phase driving coil 1. NPN-type source-side transistor 4v and sink-side transistor 5V are connected in series between the power source Vcc and the ground Vss, and an emitter-collector connection therebetween is connected to the released-side end of the V-phase driving coil 2. NPN-type source-side transistor 4W and sink-side transistor 5W are connected in series between the power source Vcc and the ground Vss, and an emitter-collector connection therebetween is connected to the released-side end of the W-phase driving coil 3.
FIG. 3 is a diagram showing waveforms of a rotating motor.
Sinusoidal wave signals Uin, Vin, Win, obtained from the Hall elements are amplified, and then converted into logic signals Uulogic, Vlogic, Wlogic, each having three value levels, namely, high, middle and low. The source-side transistors 4U, 4V, 4W are switched on in response to an H-level logic signal, while the sink-side transistors 5U, 5V, 5W are switched on in response to an L-level logic signal. That is, during the period 1, or the first period of the hexa-divided one cycle of each of the logic signals Ulogic, Vlogic, Wlogic, the source-side transistor 4U and the sink-side transistor 5V are switched on, so that a driving current a flows. During the period 2, the source-side transistor 4U and the sink-side transistor 5W are switched on, so that a driving current b flows. During the period 3, the source-side transistor 4V and the sink-side transistor 5W are switched on, so that a driving current c flows. During the period 4, the source-side transistor 4V and the sink-side transistor 5U are switched on, so that a driving current d flows. During the period 5, the source-side transistor 4W and the sink-side transistor 5U are switched on, so that a driving current e flows. During the period 6, the source-side transistor 4W and the sink-side transistor 5V are switched on, so that a driving current f flows. These driving currents a, b, c, d, e, f, being sequentially supplied to the driving coils 1, 2, 3, generally cause the brushless motor to rotate in a forward direction. Note that, for driving the brushless motor at a constant speed, the sink-side transistors 5U, 5V, 5W are given PWM (pulse width modulation) control. Specifically, when the current rotation speed of the brushless motor is slower than the constant rotation speed, PWM control with high on-duty is applied. On the other hand, when the current rotation speed of the brushless motor is faster than the constant rotation speed, PWM control with high off-duty is applied.
FIG. 4 is a circuitry diagram including major elements, showing the state of flowing driving current d. Note that a driving coil 6 is a simplified representation of a serial body consisting of U-phase and V-phase driving coils 1, 2. Regenerative diodes 7U, 8U, 7V, 8V are for regenerating and absorbing kick-back voltage caused in the driving coil 6. A smoothing capacitor 9 smoothens ripples imposed on the power source Vcc. A current detection resistor 10 converts an output current of a source-side and sink-side transistors which then remain in an ON state. Specifically, a voltage between both ends of the current detection resistor 10 is compared with a reference voltage. When the detected voltage is higher than a reference voltage, it is determined that an excessive current, or a current higher than a regular value, is supplied to the source-side and sink-side transistors. Based on such a detection result, one of sink-side transistors that is under PWM control is switched off as current limit function.
To control the brushless motor, inverted (reverse) currents of the driving currents a, b, c, d, e, f, namely inverted driving currents *a, *b, *c, *d, *e, *f, are sequentially supplied to the driving coils 1, 2, 3. Specifically, in the case of FIG. 4, the source-side transistor (4V) and the sink-side transistor (5U), which then remain in an ON state, are switched off, and the source-side transistor (4U) and the sink-side transistor (5V) are switched on to thereby flow driving current *d, indicated by the broken line.
FIGS. 5A and 5B are waveform diagrams illustrating the relationship between a counter electromotive voltage and a driving voltage for any one of the U-phase, V-phase, and W-phase coils when the brushless motor is rotated in either a forward or reverse direction. In order to rotate the brushless motor in a forward direction, a source-side transistor of the concerned phase is switched on during a period corresponding to an electrical degree 120xc2x0 with a positive voltage waveform, and a sink-side transistor of that phase is switched on during a period corresponding to an electrical degree 120xc2x0 with a negative voltage waveform (FIG. 5A). In U-phase example, in order to rotate the brushless motor in a forward direction, an output current equivalent to a counter electromotive voltage divided by a resistance value of the driving coil 6, indicated by the slanting lines, may flow through the source-side and sink-side transistors 4U, 5U.
On the other hand, in order to rotate the brushless motor in a reverse direction, a sink-side transistor of the concerned phase is switched on during a period corresponding to an electrical degree 120xc2x0 with a positive voltage waveform, and a source-side transistor of that phase is switched on during a period corresponding to an electrical degree120xc2x0 with a negative voltage waveform (FIG. 5B). That is, when a brushless motor being in a forward rotation is changed to rotate in a reverse direction for control application, in the case of U-phase, for example, an output current, indicated by the slanting lines, may flow through the source-side and sink-side transistors 4U, 5U, which is larger than that which would flow therethrough in a forward rotation. As the voltage between the two ends (a both end voltage) of the current detection resistance 10 receiving an output current when the motor is rotating in a reverse rotation is larger than a reference voltage, a current limit function is applied to turn off the sink-side transistor 5U.
When the source-side transistor 4U is switched on and the sink-side transistor 5V is switched off under the above current *d conditions, the sink-side transistor 5V is caused to switch off because the output current is too large. The source-side transistor 4U, however, remains in an ON state. As a result, a regenerative current *d continues to flow through a close loop (*d loop), including the source-side transistor 4U, the driving coil 6, and the regenerative diode 7V. As a large current then flows into the source-side transistor 4U, the source-side transistor 4U may be forced to face the risk of being destroyed. A similar problem would be caused by the flow of driving currents a, b, c, e, f.
In light of this problem, according to the present invention, there is provided a motor driving circuit for preventing the continuous flow of a large current into an output transistor when motor control is applied.
According to the present invention, source-side and sink-side transistors are repeatedly switched on and switched off when a large motor driving current is applied. With this arrangement, a current to flow through the both transistors can be limited. In particular, when braking the motor by providing a reverse drive current, a large current may be caused and, according to a method in which a sink-side transistor is switched on in such a situation, there may be a risk that a large current may flow through the source-side transistor. The present invention can prevent such a risk.